We now have information about the plot of the third A:TLA comics series, The Rift. From Dark Horse:

"Avatar Aang asks his friends to help him honor Yangchen's Festival -- one of the highest Air Nomad holidays, which hasn't been celebrated in over one hundred years.

But cryptic visits from the spirit of Avatar Yangchen herself lead Aang to discover a jointly owned Fire Nation and Earth Kingdom refinery -- operating on land sacred to the Airbenders! Is this the cause of the pollution Aang finds there, or is a more mysterious menace afoot?"

Very interesting! We know very little about Avatar Yangchen, so it will be nice to learn more about her.

The Rift will be released on March 5th, 2014, and can already be pre-ordered on Amazon.